Interferometer for integrated optical circuits.

Abstract

An interferometer (100) for an optical integrated circuit includes a first waveguide (104), a second waveguide (108), and a layer (106) in at least one of the first waveguide (104) or the second waveguide (108). The first waveguide (104) has a first effective refractive index and a first path length along an optical propagation axis of the first waveguide (104). The second waveguide (108) has a second effective refractive index and a second path length along an optical propagation axis of the second waveguide (108). The interferometer (100) is configured to reduce a change in difference between the first path length multiplied by the first effective refractive index and the second path length multiplied by the second effective refractive index. The change in difference is caused by at least one of an expansion force or a contraction force from the layer (106).

Description

[Background technology]

[0001] Interferometers are used to modulate the intensity and / or wavelength of an optical signal, which allows control of the light intensity at the optical output, possibly as a function of wavelength. It is desirable to have more precise control over the wavelength response and / or intensity of the optical signal.

[0002] It would be desirable to provide an improved interferometer for use in photonic integrated circuits (PICs). [Brief description of the drawings]

[0003] [Figure 1] 1 illustrates a schematic cross-sectional view of an interferometer according to an example; [Diagram 2] 4A and 4B show schematic diagrams of interferometers according to further examples; [Diagram 3] 3 shows a schematic cross-sectional view of the interferometer of FIG. 2 taken at the plane P200. [Figure 4] 3 shows a schematic cross-sectional view of the interferometer of FIG. 2 taken at plane P210. [Diagram 5] 4A and 4B show schematic diagrams of interferometers according to further examples; [Figure 6] 6 shows a schematic cross-sectional view of the interferometer of FIG. 5 taken at plane P320. [Figure 7] 1 shows a schematic cross-sectional view of a further example interferometer; [Figure 8] 4A and 4B show schematic diagrams of interferometers according to further examples; [Figure 9] 4A and 4B show schematic diagrams of interferometers according to further examples; [Figure 10] 1 shows a schematic plan view of an integrated optical circuit according to a further example; [Figure 11] 1 illustrates a method of manufacturing an interferometer according to an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0004] An interferometer has two waveguides, and the path length or effective refractive index difference between the two waveguides is designed to produce a desired phase difference between the light propagating through the waveguides and the resulting interference. In certain circumstances, the inventors have observed this difference to change to such an extent that the performance of the interferometer is unacceptable. It is now recognized that the planarization layer (e.g., polyimide) can expand or contract, for example in response to changes in temperature, to an extent that impairs the performance of the interferometer. Even when the expansion or contraction of the layer is constrained (e.g., by attachment to adjacent structures), the planarization layer can still exert forces on adjacent structures that affect the effective refractive index of the waveguide.

[0005] In situations where the first waveguide of the interferometer has a longer path length than the second waveguide of the interferometer, for example, an asymmetric Mach-Zehnder interferometer (asymmetric MZI), there may be a different change in the effective refractive index of the first waveguide compared to the change in the effective refractive index of the second waveguide. This changes the difference (for each waveguide), the product of the effective refractive index multiplied by the path length, the phase difference, and the time of flight, impairing the performance of the interferometer. The change in difference may shift the wavelength of the output of the interferometer by, for example, 0.2 nanometers. In applications such as long distance communications, the operating wavelength band may be, for example, 0.8 nm, so such a shift in the output wavelength adversely affects the performance of the device.

[0006] One approach to improving the interferometer would be to remove layers that can expand and contract, but this is less desirable because, while a planarizing layer can be used to protect the waveguide from external contamination, the planarizing layer also provides planarization and other functions to the interferometer.

[0007] Our insight is how to configure an interferometer such that such forces (herein referred to as expansion and contraction forces) do not unduly affect the performance of the interferometer, especially given the sensitivity of the performance of the interferometer (e.g., asymmetric MZI) to such forces. For example, the output wavelength of the interferometer changes by less than + / - 0.1 nanometers, for example, when used as a passive wavelength filter. In doing so, we realize various techniques to at least partially modulate the generation of the forces, at least partially modulate (directly or indirectly) the transfer of such forces to one or more structures that affect the effective refractive index of the waveguides, and / or at least partially compensate for the effect of the forces on the difference in effective refractive index between the waveguide(s) and thus the output of the interferometer. This is not trivial when comparing very small scale PICs to large scale chips, and indeed the challenges described herein do not manifest themselves in large scale systems, or at least have negligible effects.

[0008] Accordingly, the inventors have devised an example of an improved interferometer that retains the benefits of structures susceptible to such expansion and contraction forces (e.g., planarization layers) while still having the desired performance of a PIC even as the interferometer ages (layer materials may tend to expand over time) and / or in adverse conditions (e.g., temperature changes in the range of 0° C. to 70° C., humidity changes in the range of 5% relative humidity to 95% relative humidity, air pressure changes in the range of 200 kilopascals for suction in space, dust, radiation (background and nuclear), exposure to ultraviolet light (during assembly), and reflow soldering exposure (during assembly)).

[0009] These examples are explained in more detail below.

[0010] Examples of interferometers for PICs are described herein. Such interferometers are configured to reduce the impact of expansion or contraction forces applied by a structure (e.g., a layer such as a planarization layer) on the performance of the interferometer. Examples of such interferometers include a first waveguide having a first effective refractive index and a first path length along the optical propagation axis of the first waveguide, a second waveguide having a second effective refractive index and a second path length along the optical propagation axis of the second waveguide, and a layer on at least one of the first waveguide or the second waveguide. The interferometer is configured to reduce a change in the difference between a) the first path length multiplied by the first effective refractive index and b) the second path length multiplied by the second effective refractive index. The change in the difference is caused by at least one of the expansion or contraction forces described above.

[0011] The waveguide(s) do not necessarily have to be subjected to at least a portion of the force(s) for the effective refractive index to be affected, but if the waveguide(s) are subjected to at least a portion of the force(s), the effective refractive index and therefore the performance of the interferometer will be affected. Thus, in some examples, the interferometer is configured to reduce the expansion or contraction force imparted from the layer to at least one of the first waveguide or the second waveguide, which in turn reduces the effect of the force(s) on the performance of the interferometer.

[0012] As will become more apparent from the examples described below, the interferometer is configured to reduce the change in the difference between a) and b) in various ways, including at least one of: i) compensating for the effect of the force(s) on the change in the difference, e.g., by mechanically and / or optically compensating for the effect of the force(s) on the change in the difference; ii) at least partially modulating the generation of the force(s), e.g., by at least partially mechanically modulating the generation of the force(s); or iii) at least partially modulating the transmission of the force(s) to cause the change in the difference, e.g., by at least partially mechanically modulating the transmission of the force(s) to cause the change in the difference.

[0013] As will be appreciated, compensating may be considered as providing an action that counters, e.g., at least partially cancels, or at least partially corrects, an undesirable effect, such as a change in the performance of the interferometer caused by the force(s). Optical compensation is performed, for example, using a compensating optical action, and mechanical compensation is done, for example, with a compensating mechanical feature or structure of the interferometer. Sometimes the mechanical feature or structure may in turn provide a corrective optical action.

[0014] Modulating can be, for example, an adjustment or change to the generation and / or transmission of the force(s), thereby reducing the impact on the performance of the interferometer.

[0015] Next, an example of reducing the variation of the difference between a) and b) will be described. It should be understood that the different features described using different figures may be used in combination in further examples. For example, the width of the waveguide is selected to reduce the variation of the difference between a) and b), as well as having a recess in the layer.

[0016] Referring now to FIG. 1, in an example, an interferometer 100 is for an optical integrated circuit, the interferometer comprising a first waveguide 104 having a first effective refractive index and a first path length along the optical propagation axis OPA 100 of the first waveguide, a second waveguide 108 having a second effective refractive index and a second path length along the optical propagation axis OPA 100 of the second waveguide, and a layer 106 on at least one of the first or second waveguides. The layer extends over the first and second waveguides such that in these examples, portions of the layer contact the first and second waveguides, respectively. In these and other examples described herein, portions of the layer directly contact the surface of the waveguide, and in other examples, portions of the layer are supported by the waveguide. In these ways, at least a portion of the layer is considered to be on the waveguide, e.g., the surface of the waveguide furthest from the substrate 102 on which the waveguide is formed, or on a side of the waveguide. Thus, because of the presence of a layer in the waveguide, expansion or contraction forces (e.g., stress, strain, shear, compression, or tension) from the layer can be transferred to one or both of the waveguides. In the example of Figure 1, part of the layer is also between the waveguides, but in other examples, alternative layers or materials may be between the waveguides. The layer may be, for example, a polymer dielectric or include polyimide, polyacrylate, polycarbonate, polysiloxane, or benzocyclobutene.

[0017] In an example such as that of FIG. 1, the interferometer 100 is configured to reduce the change in the difference between a) and b) by appropriate design of the waveguides, including the width of the waveguides. As shown, the width W102 of the first waveguide 104 perpendicular to the optical propagation axis OPA 100 of the first waveguide is different from and parallel to the width W104 of the second waveguide 108 perpendicular to the optical propagation axis OPA 100 of the second waveguide. The width of at least one of the first or second waveguides is, for example, 1.5 micrometers, 13 micrometers, or between 0.2 micrometers and 20 micrometers. If, for example, one waveguide is wider than the other, a properly selected width of the waveguides may reduce the effect of the force(s). This is because the structure of such a wider waveguide is less likely to be deformed by the force(s), for example because it is mechanically stiffer. It should be noted that in the illustrated example, the width is shown in one orientation, but in further examples, the width may be obtained in a different orientation perpendicular to the propagation axis (e.g., an orientation that may be considered as the height of the waveguide). In some examples, multiple such widths (e.g., widths perpendicular to each other) are established and the waveguide is dimensioned to reduce the effect of the force(s).

[0018] It should be noted that because one waveguide has a longer optical path length than the other, the action of the force(s) on one of the waveguides will be different compared to the other, thus resulting in a change in the difference between a) and b). To reduce this change in difference, the width of the first waveguide is selected to be different from the width of the second waveguide, e.g., compensated differently for each waveguide for the action of the force(s) on each waveguide. In other words, if one of the waveguides is more affected by the force(s) than the other, the former will require more compensation than the latter to maintain the desired difference between a) and b). This can be mathematically thought of as follows for an asymmetric interferometer, although such description should not be construed as limiting, at least for some examples. The first waveguide has a path length L1 that is longer than the path length L2 of the second waveguide by a path length L3. If such compensation of the force(s) (or, for example, modulating the generation or transmission) is optimized, the effect of the change in the force(s) on the performance of the interferometer may theoretically be negligible. For example, if the force(s) only affect the effective refractive index of the first and second waveguides, the effect on the first waveguide is the change in the effective refractive index of the first waveguide, DN1, multiplied by the path length of the first waveguide, L1, and the effect on the second waveguide is the change in the effective refractive index of the second waveguide, DN2, multiplied by the path length of the second waveguide, L2. The changes are then compensated for if the following condition is met:

number

[0019] This condition assumes that the respective change in effective refractive index is uniform across each waveguide and does not change over the length of any of the waveguides.

[0020] In other examples, such as those of Figures 2-4, the width of the waveguide can be used to compensate for the force(s), where a first width W202 of the first waveguide 204 is perpendicular to the optical propagation axis of the first waveguide OPA 200 at a first point of the optical propagation axis of the first waveguide, and is different from and parallel to a second width W212 of the first waveguide, which is perpendicular to the optical propagation axis of the first waveguide at a second point of the optical propagation axis of the first waveguide. This change in width along the axis of the waveguide allows the waveguide to be considered tapered, where the change in width is, for example, stepped or stepped, the latter resulting in relatively low optical loss. In these examples, one waveguide has a different width along its optical propagation axis, while in other examples, both waveguides each vary in width along their respective optical propagation axes.

[0021] Features of those examples are similar to those previously described and are referred to using the same reference numerals incremented by 200 instead of 100, and the corresponding descriptions for such features apply here as well. The interferometer shown in Figures 2-4 includes a passivation layer 215 covering and in contact with the waveguides. The passivation layer improves the performance of at least one of the first and second waveguides, for example by reducing optical or electrical interference or crosstalk. The passivation layer includes, for example, silicon oxide or silicon nitride, with other materials being envisioned. Over the passivation layer is layer 206, which overlies the waveguides with portions of layer 206 each supported by the waveguide. Although a passivation layer is shown in Figures 2-4, it should be understood that other examples described herein may include a passivation layer, etc.

[0022] In further examples such as Figures 5 and 6, interferometer 300 is configured to reduce the difference change between a) and b) by a recess 314 in layer 306. Features of those examples are similar to those described above and are referred to using the same reference numerals incremented by 300 instead of 100 or 200, and the corresponding descriptions for such features apply here as well.

[0023] The recess is, for example, a thinning, recess, or opening in the layer 306, which modulates, for example, reduces the transmission of the expansion force(s) and / or contraction force(s) from one part of the layer to another part of the layer, which in turn modulates the transmission of the force(s) to one or more structures of the interferometer, which in turn affects the effective refractive index of one or more of the waveguides. In examples where the recess is an opening through the layer, the recess may also extend towards the substrate in which the waveguides are formed. Such deeper recesses may be formed by etching. The recess may, for example, be considered as a groove or channel, elongated and thus having sides corresponding to at least the surface of the layer. The width and length (e.g., lateral length) of the recess, and / or the depth are selected to provide the desired modulation of the transmission of the force(s). Furthermore, the position of the recess relative to one or both of the waveguides may be selected to improve the modulation of the transmission of the force(s), which in turn reduces the change in the difference between a) and b). In this regard, the recesses may be spaced apart by 1-5 or more times the height of the waveguide so that the material between the recesses and the waveguide is not too thin.

[0024] In examples such as those of Figures 5 and 6, the recess 314 is elongated and parallel at least in a portion of the first light propagation axis OPA 300. The interferometer includes one recess, in examples such as those of Figures 5 and 6, the recess is the first recess 314, and the interferometer 300 is configured to reduce the differential change by a second recess 316 in the layer 306. The second recess is elongated and parallel to the second light propagation axis OPA 300. The first waveguide 304 is between the first recess 314 and the second recess 316, while in other examples the second waveguide, or both the first and second waveguides, are between the first and second recesses. At least one of the first and second waveguides 304 and 308 is curved, and in some examples each of the first and second waveguides is curved. The curvature of at least one of the first or second waveguides, for example, reduces at least one of the size or footprint of the interferometer. Also, the curvature of at least one of the waveguides simplifies designs, for example, where the length of the first path is not equal to the length of the second path. If each waveguide is curved, the radius of curvature of the first waveguide 304 may be different from the radius of curvature of the second waveguide 308, while in other examples, the radius of curvature of the first waveguide is the same as the radius of curvature of the second waveguide. Correspondingly, one or more recesses may be curved corresponding to one or more of the waveguides.

[0025] In further examples, such as that of FIG. 7, the interferometer 400 is configured to reduce the difference variation by having a first distance w412 between a surface of the first waveguide 404 closest to the first recess 414 and a surface of the first recess 414 closest to the surface of the first waveguide 404 different from a second distance w416 between a surface of the second waveguide 408 closest to the second recess 416 and a surface of the second recess 416 closest to the surface of the second waveguide 408. In other words, in examples, such as that of FIG. 7, the one or more recesses are positioned such that one of the waveguides is closer than the other of the waveguides. In this manner, the recesses may modulate the transfer of force(s) to one of the waveguides more than the other waveguide. Thus, if a force(s) is affecting the effective refractive index of one waveguide more than the other, the recess can, for example, by properly positioning one of the waveguides closer to the other, compensate for the waveguide more affected by the force(s) over the other waveguide, thereby reducing the change in the difference between a) and b).

[0026] As shown in FIG. 7, the first waveguide 404 is between the first recess 414 and the second recess 416, although in other examples it is contemplated that the second waveguide, or both the first and second waveguides, are between the first and second recesses.

[0027] In examples such as that of FIG. 8, the interferometer 500 is configured to reduce the differential change by including a first recess 514 and a second recess 516, with the longer waveguide (first waveguide 504) being between the first recess 514 and the second recess 516. The interferometer comprises two power couplers (sometimes called power splitters) 520 and 522, which split the light input to the interferometer and at least one of injecting the split light into both the first and second waveguides, or combining and / or interfering the outputs from the first and second waveguides. In some such examples, the power combiner is a multimode interferometer (MMI), however, other power combiners are envisioned, such as fused waveguides or beam splitters. Each waveguide is curved, with the radius of curvature of the first waveguide 504 being different from the radius of curvature of the second waveguide 508. This simplifies the design of, for example, an interferometer 500 in which the path length of the first waveguide is different from the path length of the second waveguide. Features of those examples are similar to those described above and are referred to using the same reference numerals incremented by 500 instead of 100, 200, 300, or 400, and the corresponding descriptions for such features apply here as well.

[0028] In examples such as that of FIG. 9, the interferometer 600 is configured to reduce the differential change with a 1×1 MMI 624 in the first waveguide 604 (1×1 refers to the number of input and output ports of the MMI, e.g., an MMI with three inputs and three outputs is a 3×3 MMI). Other configurations are envisioned, e.g., an MMI in the second waveguide, or an MMI in both the first and second waveguides. Other MMIs are envisioned, e.g., 2×2, 2×1. In some such examples, the MMI reduces the effect of force(s) on the effective index of the first waveguide because the MMI is a larger structure and therefore may be more resistant to mechanical forces than the waveguide. An exemplary waveguide width is 1.5 micrometers and an exemplary 1×1 MMI width is 13 micrometers, although other widths are envisioned. The interferometer comprises two power couplers 620 and 622, which split the light input to the interferometer and at least one of injecting the split light into both a first and a second waveguide, or combining and / or interfering the outputs from the first and second waveguides. The example features are similar to those described above and are referred to using the same reference numerals incremented by 600 instead of 100, 200, 300, 400, or 500, and the corresponding descriptions for such features apply here as well.

[0029] In an example, such as that of Figure 10, the PIC includes an interferometer 700. The interferometer 700 resides in a monolith 730, e.g., a substrate of the PIC. The monolith may be, for example, a compound of elements from groups III and V of the periodic table, e.g., the so-called III-V semiconductor compounds, such as indium phosphide (InP), but also gallium arsenide (GaAs), gallium antimonide (GaSb), gallium nitride (GaN), indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), aluminum gallium indium aluminum gallium arsenide (InAlGaAs), indium gallium arsenide phosphide (InGaAsP), silicon (Si), silicon nitride (Si3N4), or silicon oxide (SiO2), although other semiconductor, semi-insulating, and insulating materials are envisioned. Features of those examples that are similar to features described above will be referenced using the same reference numerals incremented by 700 instead of 100, 200, 300, 400, 500, or 600, and the corresponding descriptions for such features apply here as well.

[0030] In some examples, the device includes the PIC described above. The device is, for example, a laser, a photodiode, a photovoltaic, a communications device, a light detection and ranging (LiDAR) device, a distributed Bragg reflector device, or an optical imaging device.

[0031] In some examples, a PIC includes two or more interferometers that can be optically coupled to provide functions such as both wavelength and intensity modulation, or to provide output light having a narrower wavelength dispersion than can be achieved with a single interferometer.

[0032] In some examples, each of the first and second waveguides has the same structure and is therefore formed from the same material. For example, each of the waveguides is formed, for example, including InP. Using such a semiconductor material with strong electro-optical effects, low electrical and optical losses can be achieved for a universal platform.

[0033] In some examples, modulating the phase difference between the output of the first waveguide and the output of the second waveguide is accomplished by electro-optically modulating a potential difference applied to at least one of the first waveguide or the second waveguide, or the difference between the potential difference applied to the first waveguide and the potential difference applied to the second waveguide. In some such modulators, at least one of the first waveguide or the second waveguide comprises an electrorefractive modulator. In some examples, the electrorefractive modulator is an MZI or an asymmetric MZI. The MZI or the asymmetric MZI provides a fast modulation speed and a large optical extinction. In some examples, a potential difference is applied across the first waveguide by a first electrode of the first waveguide, or a potential difference is applied across the second waveguide by a second electrode of the second waveguide. As one skilled in the art will recognize, each electrode comprises, for example, is formed of a sufficiently highly conductive material, such as gold (Au). In other examples, other conductors are contemplated, such as silver (Ag), platinum (Pt), nickel (Ni), carbon (C), cadmium (Cd), tungsten (W), aluminum (Al), or copper (Cu).

[0034] It should be appreciated that at least one of, for example, a semiconductor, a dielectric, or air is present between the first and second waveguides. This separation of the first and second waveguides helps reduce optical interference between the first and second waveguides and reduces electrical crosstalk between the first and second waveguides.

[0035] As one skilled in the art will appreciate, an interferometer uses interference between multiple light beams when the light beams are mixed. A phase difference between the light beams results in constructive or destructive interference. In some examples, the interferometer is an MZI, such as an asymmetric MZI. Other exemplary interferometers are, for example, a Fizeau interferometer, a Fabry Perot interferometer, a Michelson interferometer, or a Lyot interferometer. In some examples, the interferometer includes an electrorefractive modulator. In some such examples, the interferometer is configured for light including infrared radiation. In some such examples, the wavelength of the light is between 1200 nanometers and 1700 nanometers and / or is used for telecommunications applications. In examples, the interferometer as a component of a PIC includes a stack of structures, layers, and / or features that work together as, or at least partially contribute to, the function of the interferometer.

[0036] A PIC (photonic integrated circuit) can be thought of as an optical circuit in which two or more optical devices (e.g., devices that use light to perform functions) are optically connected. A PIC can include components that perform functions including, for example, emitting light, detecting light, amplifying light, guiding light, reflecting light, diffracting light, mixing different wavelengths of light, separating different wavelengths of light, polarization conversion, optical filtering, power division, amplitude modulation, and phase modulation. A PIC can be thought of as an optical analog of an electronic circuit. PICs typically operate in the visible or near infrared regions of the electromagnetic spectrum (e.g., light having wavelengths ranging from about 400 nanometers (nm) to about 1650 nm). However, PICs can operate in other regions of the electromagnetic spectrum. In some examples, the light includes infrared radiation. In some such examples, the wavelength of the light output from the interferometer is tunable. In some examples, such wavelengths are used in telecommunications applications.

[0037] A commercially available material platform for PICs is indium phosphide (InP), which allows for the integration of optically active and passive functions on the same chip, but other material platforms such as silicon (Si), gallium arsenide (GaAs), or lithium niobate (LiNBO3) can be utilized as platforms for PICs. A PIC can contain hundreds of components integrated on a single chip.

[0038] Those skilled in the art will recognize that a waveguide is for guiding light. Light propagates in the waveguide and is confined in the waveguide due to reflections at the boundaries of the waveguide. The waveguide has a higher refractive index than the refractive index of the material in contact with the waveguide at the boundaries where it is desired to confine the light. For example, due to this difference in refractive index at the boundaries where it is desired to confine the light, total internal reflection occurs when the angle of incidence at those boundaries of the waveguide is greater than the critical angle. In this way, the waveguide guides the propagation of light. For a particular optical mode to propagate in the waveguide, it is desirable for the light reflected at the boundaries of the waveguide to meet the conditions for constructive interference. Quantum wells are commonly used in semiconductor structures to confine electrons. For example, in optoelectronic devices, quantum wells are used to tune the electronic band gap, thereby tuning the energy (or frequency) of photons emitted (in the case of an emitter) or absorbed (in the case of an absorber). A multiple quantum well waveguide is a waveguide that includes multiple quantum wells and is envisioned as an example waveguide described herein. In many applications, the effect of a quantum well can be increased by providing multiple quantum wells in the stack.

[0039] The effective refractive index is a dimensionless number that describes how fast light travels through a waveguide and how much light is attenuated through the waveguide. The effective refractive index is generally expressed as a complex number. However, in this specification, only the real component of the refractive index is considered. In some examples, the effective refractive index depends on the wavelength of light considered. Now, when making a comparison between two refractive indices, the comparison is between the real components for the same wavelength of light.

[0040] The optical propagation axis of a waveguide is the axis of the waveguide along which the waveguide is configured for light to propagate. In some examples, the optical propagation axis is curved, for example due to curvature of the waveguide.

[0041] The path length of a waveguide is the distance that light propagating through the waveguide travels as it propagates along the optical propagation axis of the waveguide. In some instances, the path length is affected by the width of the waveguide and / or the cladding of the waveguide. Path length is otherwise known as the physical path length and is not used as an equivalent to optical path length; optical path length is generally interpreted as the path length (or physical path length) multiplied by the refractive index of the medium through which the path passes.

[0042] Although the layers referred to herein may be, for example, a single layer of homogenous material, in other examples it is contemplated that a layer may include one or more sub-layers, each of a different material.

[0043] 11 shows a schematic diagram of a method for fabricating an interferometer according to an example, including at least partially forming a first waveguide, at least partially forming a second waveguide, and at least partially forming a layer. Those skilled in the art will readily understand how to fabricate the example interferometer described herein using techniques such as, for example, metal organic vapor deposition, surface passivation, photolithography, ion implantation, etching, dry etching, ion etching, wet etching, buffered oxide etching, plasma ashing, heat treatment, annealing, thermal oxidation, chemical vapor deposition, atomic layer deposition, physical vapor deposition, molecular beam epitaxy, laser lift-off, electrochemical deposition, electroplating, or chemical mechanical polishing. In some examples, as those skilled in the art will understand, etching techniques are used to remove portions of layers, for example to provide recesses.

[0044] Examples include a method of designing an interferometer according to the examples, where the interferometer is configured to reduce a change in a difference between a first path length multiplied by a first effective refractive index and a second path length multiplied by a second effective refractive index, the change being caused by at least one of an expansion or contraction force from the layer. In this manner, the interferometer can be designed with appropriately shaped and sized features, such as a waveguide width, to compensate for the contraction or expansion force from the layer.

[0045] The above examples should be understood as illustrative examples of the present invention, further examples of which are envisioned.

[0046] It will be understood that any feature described in connection with any one of the examples may be used alone or in combination with the other features described, and also in combination with one or more other features of any of the examples, or in any other combination of any of the examples. Moreover, equivalents and modifications not described above may be employed without departing from the scope of the appended claims.

Claims

1. 1. An interferometer for an optical integrated circuit, comprising: a first waveguide having a first effective refractive index, the first waveguide having a first path length along an optical propagation axis of the first waveguide; a second waveguide having a second effective refractive index and a second path length along an optical propagation axis of the second waveguide; a layer over at least one of the first waveguide or the second waveguide; The interferometer comprises: the first path length multiplied by the first effective refractive index; configured to reduce a change in the difference between the second path length multiplied by the second effective refractive index; The change in the difference is caused by at least one of an expansion or contraction force from the layer.

2. The interferometer of claim 1 configured to reduce application of the expansion or contraction force from the layer to at least one of the first waveguide or the second waveguide.

3. The change in the difference is i) compensating said force action for said change in said difference; ii) at least partially modulating the generation of said force; or 3. The interferometer of claim 1 or 2, configured to at least one of: a) at least partially modulate the force transmission to cause the change in the difference; and b) at least one of modulate the force transmission to cause the change in the difference.

4. Pursuant to i), mechanically compensating the effect of the force for the change in the difference; or and optically compensating for said effect of said force on said change in said difference.

5. at least partially mechanically modulating said force generation according to ii); or and at least partially mechanically modulating said force transmission to cause said change in said difference according to iii).

6. The change in the difference is a width of the first waveguide perpendicular to the optical propagation axis of the first waveguide; a width of the second waveguide perpendicular to the optical propagation axis of the second waveguide; 6. An interferometer according to claim 1, wherein the width of the first waveguide is different and parallel to the width of the second waveguide.

7. The change in the difference is a first width of the first waveguide perpendicular to the optical propagation axis of the first waveguide at a first point on the optical propagation axis of the first waveguide; configured to decrease by a second width of the first waveguide perpendicular to the optical propagation axis of the first waveguide at a second point on the optical propagation axis of the first waveguide; 7. The interferometer of claim 1, wherein the first width of the first waveguide is different and parallel to the second width of the first waveguide.

8. An interferometer according to claim 1 , configured to reduce the change in the difference by a recess in the layer.

9. The interferometer of claim 8 , wherein the recess is elongated parallel to at least a portion of the first optical propagation axis.

10. 10. An interferometer according to claim 8 or 9, wherein the recess is a first recess and the interferometer is configured to reduce the change in the difference by a second recess in the layer.

11. 11. The interferometer of claim 10, configured to reduce the change in the difference by having a first distance between a surface of the first waveguide closest to the first recess and a surface of the first recess closest to the surface of the first waveguide different from a second distance between a surface of the second waveguide closest to the second recess and a surface of the second recess closest to the surface of the second waveguide.

12. 12. An interferometer according to claim 10 or 11, wherein the second recess is elongated parallel to the second optical propagation axis.

13. 13. An interferometer according to claim 10, wherein at least one of the first waveguide or the second waveguide is between the first recess and the second recess.

14. 14. An interferometer according to any one of claims 1 to 13, configured as a passive wavelength filter.

15. 15. An interferometer as claimed in any one of claims 1 to 14, wherein the change in wavelength output of the interferometer due to a change in expansion or contraction force from the layer is less than or equal to 0.1 nanometers.

16. 16. An interferometer according to claim 1, comprising a passivation layer between the layer and at least one of the first waveguide or the second waveguide.

17. 17. The interferometer of claim 1, wherein at least one of the first waveguide or the second waveguide comprises an electrorefractive modulator.

18. 18. The interferometer of claim 1, wherein at least one of the first waveguide or the second waveguide is a multiple quantum well waveguide.

19. 19. The interferometer of any one of claims 1 to 18, wherein the interferometer is an asymmetric Mach-Zehnder interferometer.

20. 20. The interferometer of claim 1, wherein the at least one of the expansion or contraction forces is due to at least one of changes in humidity, temperature, or age of the layer.

21. The interferometer of claim 1 , wherein the layer comprises a polymer dielectric.

22. 22. The interferometer of claim 1, wherein the layer comprises a polyimide, a polyacrylate, a polycarbonate, a polysiloxane, or a benzocyclobutene.

23. 23. The interferometer of claim 1, wherein at least one of the first waveguide or the second waveguide is curved.

24. 24. The interferometer of claim 1, wherein each of the first waveguide and the second waveguide is curved, the radius of curvature of the first waveguide being different from the radius of curvature of the second waveguide.

25. 25. An interferometer according to any preceding claim, comprising indium phosphide.

26. 26. The interferometer of claim 1, wherein the interferometer is on a monolith of an integrated optical circuit.

27. 27. An optical integrated circuit comprising an interferometer according to any one of claims 1 to 26.

28. An optical integrated circuit comprising two or more interferometers according to any one of claims 1 to 26.

29. 29. A device comprising an optical integrated circuit according to claim 27 or 28.

30. 1. A method comprising: at least partially forming the first waveguide; at least partially forming the second waveguide; and A method of manufacturing an interferometer according to any one of claims 1 to 26, comprising at least partially forming said layer.

31. A method of designing an interferometer configured to reduce said variation of said difference according to any one of the preceding claims.

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